How do I zoom in on a planet from space to ground level?
I would make it into few scenes that I would then merge in post-production.

Scene 1 - from space, getting close to planet, you could have sunflare, satellite or whatever cover view for a moment, so you could load Scene 2 then.

Scene 2 - Scene 2 starts exactly where Scene 1 ended, with more detailed planet done. It would be going from planet to some upper atmosphere clouds, at point in time cloud would cover view, that is the moment when you start scene 3

Scene 3 - starts exactly where Scene 2 ended, with same cloud covering the scene, but with much more detailed ground and you getting even closer.

And basically that would be it - each time you'd want to increase your details you would have to find a way to cover view for a fraction of a second to switch scenes seamlessly. This way you could do it all the way to the ground and beyond :)